FIELD OF THE INVENTION
Electrostatic transducers capable of covering the audio spectrum from 30 Hz to 20,000 Hz present a capacitive load characteristic which is very difficult to drive. Such transducers typically require a push-pull signal approximating 3000 volts r.m.s. at low frequencies and about 1000 volts r.m.s. at mid-band and high frequencies. Such transducers have a highly capacitive character typically approximating 800 picofarads. This causes the driving source to see an impedance varying from about 7 megohms to less than 10 kilohms.
Practical electrostatic speakers made their debut in the early 1950's with Arthur Janszen's small tweeter array. This innovation was successful and is still available in much the same format today.
There have been more than a few attempts at full range electrostatic design for capacitive loads driven by conventional amplifiers over the years, and they have all had three problems in common, to wit inadequate sound output levels even when driven by exceedingly high powered amplifiers, generally inadequate bass response, and distortion and non-linearity caused by the necessity of using high voltage step-up transformers.
In addition, by trying to squeeze out more level with more and more power in this inefficient arrangement, panels are frequently over-driven and burned out.
The crux of the problem lies not so much with the electrostatic speakers themselves and their panels but rather with the amplifier that powers them. A full range electrostatic system does not pose any significant resistive load but does not provide an enormous capacitive load. Conventional amplifiers are designed for resistive loads. To give such an amplifier an 800-1000 pf capacitive load (faced through a 100:1 step-up transformer to obtain the required drive voltages) is to ask for marginal performance and poor reliability.
By designing an amplifier ideally matched to the high capacitive load, the design goals of high sound pressure level, excellent bass response, unrivaled linearity, low distortion and reliability could be easily met.
It is therefore clear to anyone skilled in the art that if the disadvantages of transformer audio step-up are to be avoided, this push-pull charge can best be accomplished by an arrangement employing a pair of high-voltage tube output circuits, employing the Schmitt connection (U.S. Pat. No. 1,950,365, Mar. 6, 1934) operating in push-pull.
The circuitry of the instant invention requires a much higher open-loop gain around which feedback could be taken to achieve stability, low distortion and positive DC centering. In Beveridge (U.S. Pat. Nos. 3,773,976 and 3,668,335) the feedback only loops the tubes and thus does not satisfy the requirements of having the surplus gain of the transistor stages to provide the high levels of correction necessary to achieve the objective sought.
The open loop gain of the instant invention is about 80 dB and uses about 34 dB of negative feedback, leaving a net closed-loop gain of about 46 dB.